The automotive industry has recently been adopting LED technology in tail light, head light, and other signal light applications. In some cases a combination of LED and incandescent technology is used. In other cases, lamps use solely LEDs as sources. Tail lamps with tens or even more than a hundred LEDs are observed. In general, free-space optics have been used. Disadvantages with such an approach include the need for large cavities requiring more trunk space and the additional indirect cost associated with the generally necessary metal-working of the vehicle frame housing the lamp.
More recently, solid light guides have been proposed for the rear running light or for cosmetic purposes. In general, these solid light guides have introduced the challenges of being bulky, heavy, and not especially uniform in their light emission without the use of other optical elements such as diffusers. Such optical elements add additional complexity, cost and reduce light brightness. Light extraction from these light guides generally occurs by a change in thickness of the guide. This requires one end of the guide to be thicker leading to the increased bulk. It also provides little independent control of the direction of the emission. Alternatively, extraction can occur with structural modification of one surface or by painted dots either of which can occlude visibility through the light guide.
From a decorative stand-point, LEDs are point sources that are easily resolved at standard viewing distances. LEDs have been used in direct lit signal functions, however this point light source appearance may not be appealing to a viewer due to brightness, glare and other reasons. In addition, vehicle tail lamp assemblies contain multiple signal functions. Each of these signal functions has unique color, uniformity, and intensity requirements. The running light, which is operational during vehicle operation at night, is generally red or red-amber light. The stop light, which is activated intermittently, is often the same color as the running light but is generally more intense. The back-up light is generally white light and finally, the signal light, is generally either yellow, amber or red light.